A vehicle transmission transmits mechanical engine torque to a set of drive wheels in order to propel the vehicle through a range of output speeds. In an automatic transmission, a hydrodynamic torque converter automatically and variably couples an engine output shaft to a transmission input shaft while multiplying engine torque at lower vehicle speeds. As vehicle speed decreases, an increasing amount of slip occurs between an impeller or pump and a turbine of the torque converter, with a maximum slip level reached at zero vehicle speed.
This variable slip capability allows the engine to continue to rotate when the vehicle is idling in certain transmission states or modes, for instance in park, neutral, or in a forward or reverse drive state. In some transmission designs operating in a state similar to neutral during a drive detent position, for instance when the vehicle reaches zero output speed, when coasting down toward zero speed, upon a garage shift from park or neutral to drive, or when at standstill/idling, the transmission may automatically shift into a hydraulic neutral state. In such a state, electro-hydraulic controls regulate turbine speed relative to engine speed to reduce fuel consumption and minimize noise, vibration, and harshness. This hydraulic neutral state is referred to as neutral idle.